1. Field of the Invention
The present invention relates to a roller screw and more particularly, to such a roller screw that has different load carrying capacities at the left and right sides in the axial direction.
2. Description of the Related Art
In a precision machine, there may be a design for controlling displacement of the worktable or machining member. For high-load transmission, a roller screw may be used to achieve precision transmission so that the total volume of the machine, the manufacturing cost and the manufacturing time can be reduced. The contact between rollers and the threads of a screw and nut set is a linear contact (the contact between balls and the threads of a screw and nut set is a spot contact), the contact surface area of rollers is greater than the contact surface area of balls. The load carrying capacity is directly proportional to the contact surface area. Under a same load condition, the use of rollers as a transmission medium between the nut and the screw can greatly reduce the dimension of the roller screw (in other words, the dimension of a ball screw will be greater than the dimension of a roller screw of the same capacity), in consequence, the dimension of the facility can be relatively reduced. Further, many different types of roller screws are known. However, these conventional roller screws are practical for industrial application (due to the reasons: (a) the conventional roller screws commonly have a complicated structure and are not suitable for low-cost mass-production; (b) the performance of the conventional roller screws cannot satisfy user requirements). In a roller screw, at least one thread groove is defined between the screw and the nut for accommodating multiple rollers. Roller screw designs are known as follows:
A): A screw and a nut are arranged together and a thread groove is defined in each of the screw and the nut to accommodate multiple rollers in one same direction. This thread groove design enables the rollers to rotate in the same direction and angle, i.e., the rollers bear the load in the direction of rotation in the thread grooves during displacement of the nut. Thus, the arrangement of the rollers can simply bear the load in one direction. However, when a force is produced in the no-load direction, for example, when an axial force is produced, the rollers will slide axially relative to the thread grooves, causing friction damage and roller screw operation instability. Further, because no pre-stress can be set between the rollers and the screw/nut (because the rollers are arranged in the same direction and the outer diameter of the rollers is greater than the length of the rollers), a back lash (clearance) will be produced between the rollers and the thread grooves, lowering the positioning precision and causing vibration during operation. Therefore, a roller screw of this design is not suitable for use in a high-precision machine.
B): A screw and a nut are arranged together and a thread groove is defined in each of the screw and the nut to accommodate multiple rollers in a staggered manner. Arranging the rollers in a staggered manner can bear the load from two directions and kept the load in balance. However, because only one half of the rollers can bear the load in every direction (due to staggered arrangement), this design of roller screw has the drawbacks of low load carrying capacity and low rigidity. When increasing the load carrying capacity, the number of rollers must be relatively increased, in consequence, the length of the nut must be relatively increased.
C): A screw and two nuts are arranged together, a thread groove is defined in the screw and each of the nuts, and multiple rollers are arranged in parallel in the thread grooves. According to this design, the nuts are designed to guide rotation of the rollers at different tilting angles. This design enables the roller screw to bear the load in two different directions. Because this design of roller screw is a double-nut design, the total length of the nuts on the screw is relatively longer, shortening the stroke of the nuts on the screw. To overcome this problem, the total length of the screw must be relative increased. However, increasing the length of the screw relatively increases the manufacturing cost and the material cost. Further, the contact surfaces between the two nuts require high surface accuracy and high degree of parallelism. To achieve these requirements, an extra grinding process to grind the nuts is necessary. Further, this double-nut design complicates the installation process and requires much the installation time. Therefore, the manufacturing cost of a double-nut roller screw is high.